A voltameter or coulometer is a

scientific instrument A scientific instrument is a device or tool used for scientific purposes, including the study of both natural phenomena and theoretical research. History Historically, the definition of a scientific instrument has varied, based on usage, laws, a ...

used for

measuring Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. In other words, measurement is a process of determining how large or small a physical quantity is as compared ...

electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respecti ...

(quantity of electricity) through electrolytic action. The

SI unit The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...

of electric charge is the

coulomb The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). In the present version of the SI it is equal to the electric charge delivered by a 1 ampere constant current in 1 second and to elementary ch ...

. The voltameter should not be confused with a

voltmeter A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes negligible current from the circuit. An ...

, which measures electric potential. The SI unit for electric potential is the

volt The volt (symbol: V) is the unit of electric potential, electric potential difference ( voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). D ...

Types of voltameter

The voltameter is an

electrolytic cell An electrolytic cell is an electrochemical cell that utilizes an external source of electrical energy to force a chemical reaction that would not otherwise occur. The external energy source is a voltage applied between the cell′s two elect ...

and the measurement is made by weighing the element deposited or released at the

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction i ...

in a specified time.

Silver voltameter

This is the most accurate type. It consists of two

silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...

plates in a solution of

silver nitrate Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar caustic ...

. When current is flowing, silver dissolves at the

anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemoni ...

and is deposited at the

. The cathode is initially massed, current is passed for a measured time and the cathode is massed again.

Copper coulometer

This is similar to the silver voltameter but the anode and cathode are

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish ...

and the solution is

copper sulfate Copper sulfate may refer to: * Copper(II) sulfate Copper(II) sulfate, also known as copper sulphate, is an inorganic compound with the chemical formula . It forms hydrates , where ''n'' can range from 1 to 7. The pentahydrate (''n'' = 5), a brigh ...

, acidified with sulfuric acid. It is cheaper than the silver voltameter, but slightly less accurate.

Mercury voltameter

In this device, mercury is used to determine the amount of charges transformed during the following reaction: : + 2e- <=> Hg^\circ These oxidation/reduction processes have 100% efficiency with the wide range of the current densities. Measuring of the

quantity of electricity The word ''electricity'' derives from New Latin and ultimately Greek. It first appears in English in Francis Bacon's writings. Depending on context, the word may refer to "electric charge", "electric power" or "electric energy". Historical drift ...

(

s) is based on the changes of the mass of the mercury electrode. Mass of the

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...

can be increased during

cathodic A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in w ...

deposition of the mercury ions or decreased during the

anodic An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...

dissolution of the metal.

Sulfuric acid voltameter

The anode and cathode are

platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...

and the solution is dilute sulfuric acid.

Hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

is released at the cathode and collected in a graduated tube so that its volume can be measured. The volume is adjusted to

standard temperature and pressure Standard temperature and pressure (STP) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union ...

and the

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different element ...

of hydrogen is calculated from the volume. This kind of voltameter is sometimes called

Hofmann voltameter A Hofmann voltameter is an apparatus for electrolysis of water, electrolysing water, invented by August Wilhelm von Hofmann (1818–1892) in 1866. It consists of three joined upright cylinders, usually glass. The inner cylinder is open at the top ...

Historical derivation of the name

Faraday used an apparatus that he termed a "volta-electrometer"; subsequently Daniell called this a "voltameter".Frank A. J. L. James, (1991), ''The correspondence of Michael Faraday'', IET, , letter 872, 9/1/1836 Coulometer in A

Coulometer

A coulometer is a device to determine

s. The term comes from the unit of charge, the

. There can be two goals in measuring charge: Coulometers can be devices that are used to determine an

amount of substance In chemistry, the amount of substance ''n'' in a given sample of matter is defined as the quantity or number of discrete atomic-scale particles in it divided by the Avogadro constant ''N''A. The particles or entities may be molecules, atoms, io ...

by measuring the charges. The devices do a

quantitative analysis Quantitative analysis may refer to: * Quantitative research, application of mathematics and statistics in economics and marketing * Quantitative analysis (chemistry), the determination of the absolute or relative abundance of one or more substanc ...

. This method is called

coulometry Coulometry determines the amount of matter transformed during an electrolysis reaction by measuring the amount of electricity (in coulombs) consumed or produced. It can be used for precision measurements of charge, and the amperes even used to have ...

, and related coulometers are either devices used for a coulometry or instruments that perform a coulometry in an automatic way. Coulometers can be used to determine electric quantities in the direct current circuit, namely the total charge or a constant current. These devices invented by

Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist who contributed to the study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic inductio ...

were used frequently in the 19th century and in the first half of the 20th century. In the past, the coulometers of that type were named voltameters.

Sources

* ''Practical Electricity'' by W. E. Ayrton and T. Mather, published by Cassell and Company, London, 1911, pp 12–26

References

{{Electroanalytical Measuring instruments Electroanalytical chemistry devices